One problem in the electronics industry is proper alignment or registration such as in the manufacture of printed circuit boards. Registration is the relative position of one or more printed circuit patterns or portions thereof with respect to desired locations on the printed circuit board of another pattern on the other side of the board.
One of the last steps in the manufacture of printed circuit boards is the application of solder mask to an outside layer. Liquid Photo Imagable Solder Mask (LPISM) is applied, tack dried to remove any solvents and selectively exposed using a phototool such that specific areas can be developed off the board. Such phototools, typically composed of diazo, silver halide, quartz or chrome, are prepared based on “idealized” dimensions of circuit line placement. However, variations in actual board dimensions or the circuit line from the “idealized” dimensions are common because of rigorous processing employed in the manufacture of the boards. Using an “idealized” phototool in combination with dynamically changing boards often results in registration problems particularly between boards in laminates. Because the solder mask step is one of the last steps in the manufacture of printed circuit boards, discarded boards caused by misregistration leads to costly and inefficient manufacturing processes. Misregistration creates failure of through-hole to line connection and shorts between through-holes and isolated conductors. Further, in conventional practice workers often prepare multiple fixed phototools and manually try to find the optimum fit between phototool and board to avoid misregistration. Such a process is both inaccurate and time consuming resulting in further inefficiency of multi-layer printed circuit board manufacture.
In an attempt to address the registration problem, a hybrid inkjet solder mask imaging method was developed. In this method, the solder mask is applied in a conventional manner: the printed circuit board is pre-cleaned and then coated with the solder mask and tack-dried. A vision system then captures fiducials on the board and aligns the board and the printed images with greater accuracy than is often achieved using conventional means. The inkjet printer then prints a UV blocking material on the board, thereby defining the pattern required for the solder mask. The board is exposed to the UV radiation with the inkjetted layer becoming a sacrificial mask. The board is then developed to produce a finished board with pads or openings, such as through-holes and vias.
However, the above process has its limitations. The solder mask is applied to a heavily textured surface with areas of raised through-holes, vias and metal tracks where select portions of the solder mask have to be developed off the board. Since many through-holes and vias in the printed circuit board have large surface areas, the solder mask covering the through-holes and vias does not always fill them up and planarize the surface. Inkjet printing UV block ink into the through-holes and vias wastes large amounts of ink and makes the process difficult and time consuming. In addition, once the through-holes and vias are filled with the UV block ink it is very difficult to remove the ink from the inside of the through-holes and vias. This causes a loss of continuity in the through-holes and vias thereby generating scrap boards. Accordingly, there is still a need for improved methods of forming images on substrates, such as printed circuit boards.